This invention relates to a hydrogen-absorbing alloy, a secondary battery comprising a negative electrode containing the hydrogen-absorbing alloy, a hybrid car equipped with a secondary battery comprising a negative electrode containing the hydrogen-absorbing alloy, and an electromobile equipped with the aforementioned secondary battery.
Hydrogen-absorbing alloy has been noticed as being capable of safely and easily storing hydrogen as an energy source, and also as being useful as an energy exchange material or an energy storage material. Therefore, there have being proposed various applications of hydrogen-absorbing alloy as a new functional material. For example, hydrogen-absorbing alloy has been proposed to employ for the storage and transport of hydrogen, the storage and transport of heat, the conversion of heat energy to mechanical energy, the separation and purification of hydrogen, the separation of hydrogen isotope, a battery employing hydrogen as an active material, a catalyst in synthetic chemistry, and a temperature sensor.
Particularly, the application of a secondary battery which is capable of reversibly absorbing and desorbing hydrogen to the negative electrode of a secondary battery is extensively studied. In fact, some of secondary batteries of this kind are put to practical use. By the way, a secondary battery has been employed as a power source for various kinds of portable electronic apparatus of small size and light weight. These portable electronic apparatuses are now being increasingly enhanced in the performance, function and miniaturization thereof, so that in order to enable these portable electronic apparatuses to be operable for a long period of time, the discharge capacity per unit volume of the secondary battery is required to be increased. Additionally, it is also desired in recent years to make the secondary battery more light in weight, i.e. to increase the discharge capacity per unit weight of the secondary battery in addition to increasing the discharge capacity per unit volume thereof.
A rare earth element-based hydrogen-absorbing alloy of AB.sub.5 type is capable of reacting with hydrogen at the normal temperature and pressure, and is relatively excellent in chemical stability, so that this rare earth element-based hydrogen-absorbing alloy of AB.sub.5 type is now extensively studied as a prospective hydrogen-absorbing alloy for the secondary battery, and is actually put to practical use for a negative electrode of the secondary battery available on the market. However, the discharge capacity of this secondary battery placed on the market and provided with the negative electrode containing this AB.sub.5 type rare earth element-based hydrogen-absorbing alloy now reaches to as high as 80% or more of the theoretical capacity, so that any further increase in discharge capacity would be difficult.
By the way, there are a large number of the rare earth element-Ni based intermetallic compounds other than the aforementioned AB.sub.5 type rare earth element-based intermetallic compound. For example, Mat. Res. Bull., 11, (1976) 1241 describes that an intermetallic compound containing a larger quantity of rare earth element as compared with the AB.sub.5 type compound is capable of absorbing a larger quantity of hydrogen in the vicinity of room temperature as compared with the AB.sub.5 type compound. Further, with respect to the system wherein the A-site is constituted by a mixture comprising a rare earth element and Mg, there are known a couple of publications as explained below.
Namely, J. Less-Common Metals, 73, (1980) 339 discloses a hydrogen-absorbing alloy having a composition represented by La.sub.1-x Mg.sub.x Ni.sub.2. However, this hydrogen-absorbing alloy is accompanied with a problem that due to its high stability in relative to hydrogen, hydrogen can be hardly released therefrom, thus making it difficult to fully desorb hydrogen at the occasion of discharging the secondary battery. On the other hand, there is also a report on a hydrogen-absorbing alloy having a composition of LaMg.sub.2 Ni.sub.9 (a summary of lecture in the 120th Spring Meeting of Japan Institute of Metals, p.289 (1997). However, this hydrogen-absorbing alloy is accompanied with a problem that the degree of hydrogen absorption is relatively low.
Japanese Patent Unexamined Publication S/62-271348 discloses a hydrogen absorption electrode comprising a hydrogen-absorbing alloy represented by a general formula Mm.sub.1-x A.sub.x Ni.sub.a Co.sub.b M.sub.c, while Japanese Patent Unexamined Publication S/62-271349 discloses a hydrogen absorption electrode comprising a hydrogen-absorbing alloy represented by a general formula La.sub.1-x A.sub.x Ni.sub.a Co.sub.b M.sub.c.
However, a metal oxide-hydrogen secondary battery provided with any one of these hydrogen absorption electrodes is accompanied with a problem that it is low in discharge capacity and short in charge/discharge cycle life.
Further, International Re-publication No. WO97/03213 and U.S. Pat. No. 5,840,166 disclose a hydrogen-absorbing electrode comprising a hydrogen-absorbing alloy having a specific antiphase boundary and a composition represented by the following general formula (i). This hydrogen-absorbing alloy is said as having a crystal structure consisting of LaNi.sub.5, i.e. CaCu.sub.5 type single-phase. EQU (R.sub.1-x L.sub.x) (Ni.sub.1-y L.sub.y).sub.z (i)
wherein R is at least one element selected from La, Ce, Pr and Nd; L is at least one element selected from Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Mg and Ca; M is at least one element selected from Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, Si, V, Cr, Nb, Hf, Ta, W, B and C; and x, y and z are respectively a number satisfying the conditions of: 0.05.ltoreq.x.ltoreq.0.4, 0.ltoreq.y.ltoreq.0.5, 0.ltoreq.z&lt;4.5. PA1 wherein R is a rare earth element or a misch metal (Mm); A is at least one element selected from Mg, Ti, Zr, Th, Hf, Si and Ca; B is at least one element selected from Al and Cu; C is at least one element selected from Ga, Ge, In, Sn, Sb, Tl, Pb and Bi; and X, Y, Z, .alpha., .beta. and n are respectively a number satisfying the conditions of: 0&lt;X.ltoreq.0.3, 0.3.ltoreq.Y.ltoreq.1.5, 0&lt;Z.ltoreq.0.3, 0.ltoreq..alpha..ltoreq.1.0, 0.ltoreq..beta..ltoreq.1.0, and 0.9.ltoreq.n.ltoreq.1.1. PA1 wherein R is at least one kind of element selected from rare earth elements (which include Y); T is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M1 is at least one element selected from the group consisting of Co and Fe; M2 is at least one element selected from the group consisting of Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S; and the atomic ratios of a, b, X, Y, .alpha. and Z are respectively a number satisfying the conditions of: 0.15.ltoreq.a.ltoreq.0.37, 0.ltoreq.b.ltoreq.0.3, 0.ltoreq.X.ltoreq.1.3, 0.ltoreq.Y.ltoreq.0.5, 0.ltoreq..alpha.&lt;0.135, and 2.5.ltoreq.Z.ltoreq.4.2. PA1 wherein I.sub.2 is an intensity of a peak exhibiting a highest intensity in an X-ray diffraction using CuK.alpha.-ray; and I.sub.1 is an intensity of a peak exhibiting a highest intensity within 2.theta. of 8 to 13.degree., .theta. being Bragg angle, in the X-ray diffraction; EQU R.sub.1-a-b Mg.sub.a T.sub.b Ni.sub.Z-X M3.sub.X (3) PA1 wherein R is at least one kind of element selected from rare earth elements (which include Y); T is at least one element selected from the group consisting of Ca, Ti, Zr and Hf; M3 is at least one element selected from the group consisting of Co, Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Mo, V, Cr, Ta, Li, P and S; and the atomic ratios of a, b, X and Z are respectively a number satisfying the conditions of: 0.1.ltoreq.a.ltoreq.0.6, 0.ltoreq.b.ltoreq.0.3, 0.ltoreq.X.ltoreq.2, and 2.5.ltoreq.Z.ltoreq.4. PA1 wherein R is at least one kind of element selected from rare earth elements (which include Y) with a proviso that the content of Ce in the R is less than 20% by weight (including 0% by weight); M4 is at least one element selected from the group consisting of Mn, Fe, Al, Ga, Zn, Sn, Cu, Si, B, Nb, W, Ti, Zr, In, Mo, V, Cr, P and S; and the atomic ratios of a, X, Y, Z and .alpha. are respectively a number satisfying the conditions of: 0.15.ltoreq.a.ltoreq.0.33, 0.06.ltoreq.X.ltoreq.0.15, 0.ltoreq.Y.ltoreq.0.2, 3.15&lt;Z.ltoreq.3.55, and 0.ltoreq..alpha.&lt;0.135.
This hydrogen-absorbing alloy is manufactured by allowing a melt of the alloy represented by the general formula (i) to drop on the surface of a roll having a surface roughness of 30 to 150 .mu.m in mean maximum height Rmax, whereby cooling and solidifying the melt under cooling conditions: 50 to 500.degree. C. in supercooling degree and 1,000 to 10,000.degree. C./sec. in cooling rate, thus obtaining flakes having a thickness of 0.1 to 2.0 mm, which is then heat-treated. This publication also mentions that if the aforementioned manufacturing conditions are not met, the resultant alloy may have two phases, i.e. a LaNi.sub.5 type phase and a Ce.sub.2 Ni.sub.7 type phase, and hence it is impossible to obtain an alloy constituted by LaNi.sub.5 type single phase.
However, a secondary battery equipped with a negative electrode comprising a hydrogen-absorbing alloy having a CaCu.sub.5 type crystal structure, a specific antiphase boundary and a composition represented by the aforementioned general formula (i) is accompanied with problems that it is low in discharge capacity and short in charge/discharge cycle life.
Additionally, Japanese Patent Unexamined Publication H/11-29832 discloses a hydrogen-absorbing material represented by the following general formula (ii) and having a hexagonal system wherein a space group is P6 .sub.3 /mmc, i.e. a Ce.sub.2 Ni.sub.7 type crystal structure. EQU (R.sub.1-X A.sub.X).sub.2 (Ni.sub.7-Y-Z-.alpha.-.beta. Mn.sub.Y Nb.sub.Z B.sub..alpha. C.sub..beta.).sub.n (ii)
In this hydrogen-absorbing alloy having a composition represented by the above formula (ii), the atomic ratio of Mn/(R+A) is in the range of 0.135 to 0.825.
However, since this hydrogen-absorbing alloy is poor in the reversibility of absorption/desorption reaction, the degree of hydrogen absorption and desorption is relatively low. Accordingly, a secondary battery equipped with a negative electrode comprising this hydrogen-absorbing alloy is accompanied with problems that the reversibility of absorption/desorption reaction is poor and the discharge voltage is also low, so that the discharge capacity thereof would be reduced.